Alteration of Epitope Recognition Pattern in Ag85B and ESAT-6 Has a Profound Influence on Vaccine-induced Protection Against Mycobacterium Tuberculosis

To analyze the effect of vaccine delivery systems on antigen recognition and vaccine efficacy, we compared immune responses in mice immunized either with an adenovirus vector expressing a fusion of Ag85B and ESAT-6 or with the recombinant fusion protein in a liposomal adjuvant. Both vaccines induced high levels of antigen-specific IFN-gamma production. The adjuvanted protein vaccine induced primarily a CD4 T cell response directed to the epitope Ag85B(241-255) and gave efficient protection against subsequent Mycobacterium tuberculosis infection. In contrast, the adenoviral construct induced a strong CD8 response predominantly targeted to the epitope ESAT-6(15-29) and no significant protection against infection. Vaccination with the protein vaccine resulted in highly accelerated recall of Ag85B(241-255)-specific T cells immediately post M. tuberculosis challenge whereas the ESAT-6(15-29) epitope was barely recognized during infection. Delivery of the viral construct in cationic liposomes switched the immune response to a protective one dominated by CD4 T cells targeted to the Ag85B(241-255) epitope. These data demonstrate that the nature of the T cell response to a vaccine antigen is more important than its magnitude with respect to protective efficacy and that vaccine-mediated changes in immunodominance can result in T cell responses of limited relevance during the natural infection.